This invention relates to pneumatic radial-ply runflat tires and ways to increase the bending stiffness of sidewall wedge inserts.
Various methods have been devised for enabling the safe, continued operation of unpressurized or underpressurized vehicle tires with the intent of minimizing further damage to the uninflated tire and without simultaneously compromising vehicle handling over a distance from the location where the tire has lost its pressure to a place desired by the driver, such as a service station where the tire can be changed. Loss of tire pressure can result from a variety of causes, including puncture by a foreign object such as a nail or other sharp object piercing the pneumatic tire installed on a vehicle.
Pneumatic tires designed for sustained operation under conditions of unpressurization or underpressurization are also called runflat tires, as they are capable of being driven in the uninflated, or what would generally be called the xe2x80x9cflatxe2x80x9d condition. A conventional pneumatic tire collapses upon itself when it is uninflated and is carrying the weight of a vehicle. The tire""s sidewalls buckle outward in the circumferential portion of the tire where the tread contacts the ground, making the tire xe2x80x9cflatxe2x80x9d.
The term xe2x80x9crunflatxe2x80x9d is generally used to describe a tire that is designed such that the tire structure alone, in particular the structure of the sidewalls, has sufficient rigidity and strength to support the vehicle load when the tire is operated without being inflated. The sidewalls and internal surfaces of the tire do not collapse or buckle onto themselves, and the tire does not otherwise contain or use other supporting structures or devices to prevent the tire from collapsing.
In general, runflat tires incorporate sidewalls that are thicker and/or stiffer than a conventional non-runflat tire so that the tire""s load can be carried by an uninflated tire with minimum adverse effects upon the tire itself and upon vehicle handling until such reasonable time as the tire can be repaired or replaced. The typical methods used in sidewall thickening and stiffening include the incorporation of circumferentially disposed wedge inserts in the inner peripheral surface of the sidewall portion of the carcass, which is the region in the tire usually having the lowest resistance to deformation under vertical loading. In such runflat tire designs, each sidewall is thickened in such a way that its overall thickness is increased in the region between the bead and the tread shoulder. The inserts in each sidewall, in combination with the plies, add rigidity to the sidewalls in the absence of air pressure during runflat operation. Such reinforced sidewalls, when operated in the uninflated condition, experience a net compressive load in the region of the sidewall that is in the road-contacting portion of the tire. Also, the bending stresses on the sidewalls are such that the outer portions of the reinforced sidewalls experience tensile forces while the inner portions experience compression stresses during runflat operation.
U.S. Pat. No. 5,368,082 (""082) of Oare et al, having a common assignee with the present invention, discloses a low aspect ratio, runflat, pneumatic radial ply tire which employs special sidewall inserts to improve stiffness. Approximately six additional pounds of weight per tire was required to support an 800 lb load in this uninflated tire. This earlier invention, although superior to prior attempts at runflat tire design, still imposed a weight penalty that could be offset by the elimination of a spare tire and the tire jack. However, this weight penalty becomes even more problematic in the design of tires having higher aspect ratios. The ""082 Patent teaches a sidewall construction for runflat tires in which the tire is constructed with two plies, an inner liner and two reinforcing wedge inserts in each sidewall. The two inserts in each sidewall are disposed such that one insert is located between the two plies while the other insert is located between the inner liner and the first or innermost ply.
U.S. Pat. Nos. 5,427,166 and 5,511,599 of Willard, Jr., and assigned to Michelin Corporation disclose the addition of a third ply and the addition of a third insert in the sidewall to theoretically improve the runflat performance of a runflat tire over that of the ""082 Patent discussed above. These two Michelin patents discuss some of the load relations that occur in the uninflated condition of the tire and demonstrate that the principle disclosed in the ""082 patent can be applied to additional numbers of plies and inserts.
However, use of large amounts of rubber to stiffen the sidewall members, as in the Michelin patents previously discussed, usually increase flexure heating and lead to earlier tire failure during runflat operation. This is especially so when the tire is operated at high speeds. Therefore, one goal of runflat tire design is to minimize the number of wedge inserts used to stiffen each sidewall and reduce the total amount of elastomeric wedge insert material used in a runflat tire.
While the increased resistance to compression deflection of the multiple inserts tends to prevent the collapse of the uninflated loaded tire, the use of multiple plies and more than one reinforcing wedge insert in each sidewall, has drawbacks which include the above mentioned increase in tire weight and flexure-induced heat build up. Such designs also increase the tire""s complexity in ways that adversely affect manufacturing and quality control.
U.S. Pat. No. 3,464,477 of Henri Verdler, and assigned to Michelin Corporation discloses a pneumatic tire particularly for off-highway (OTR) use wherein the inflated tire is to be protected against damage such as cuts and abrasion to the sidewalls when the tire is used on rocky or rough ground. Although this is not a runflat design, there are useful teachings presented since the inventor has determined that sidewall damage from such OTR operation can be ameliorated by reinforcing the tire sidewalls: xe2x80x9csurprisingly . . . the sidewalls should be reinforced inwardly of the carcass plies.xe2x80x9d The disclosed reinforcement of each sidewall is a single reinforcing wedge insert (xe2x80x9celastomeric reinforcementxe2x80x9d) which has a maximum thickness at the mid height of the sidewall of the tire between about 1% and 3% of the overall maximum width of the tire, the reinforcement tapering toward its edges, that is toward the tread and toward the corresponding bead of the sidewall and extending about half of the height of the tire. The elastomeric composition is not very important other than that it should not have a hysteretic loss greater than 25%. The elastomer may contain one or more plies of elastic cords of metallic or non-metallic type arranged radially or only slightly inclined to the radial direction, for example at an angle between about 10 and 30 degrees to the radial. Suitable elastic cords for reinforcing the internal layer of the tire are polyamide cords, such as nylon, or elastic metallic cables, preferably having a modulus of elasticity less than 5000 DaN/mm2. The preferred embodiment of the sidewall reinforcement xe2x80x9clayersxe2x80x9d is summarized in claim 6 and includes xe2x80x9ca pair of plies of elastic polyamide cords, . . . the cords of one ply crossing the cords of the other ply, said layers and said plies therein terminating short of said bead and the edge of said tread adjacent to said sidewall.xe2x80x9d There is no discussion or specification of the length or position of the ends of one ply relative to the other ply in the pair of plies. The disclosed design, with specific limits on insert thickness and insert ply cord angles, provides sidewall reinforcement suitable for providing xe2x80x9csurprisingly increased resistance to damage . . . cutting or gashing by cutting objectsxe2x80x9d when operating an inflated tire on rocky terrain.
Clearly, the goal in runflat tire design is to provide a low-cost, light-weight tire that gives good runflat vehicle handling as well as good service life during runflat operation.
It is an object of the present invention to provide a pneumatic, radial runflat ply tire as defined in one or more of the appended claims and, as such, having the capability of being constructed to accomplish one or more of the following subsidiary objects.
One object of the present invention is to provide a pneumatic radial runflat ply tire with a simplified design that allows for improved manufacturing quality control.
Another object of the present invention is to provide a pneumatic radial runflat ply tire having a reduced weight as compared with prior art designs.
Another object of the present invention is to provide a pneumatic radial runflat ply tire having a reduced heat generating potential during runflat operation as compared with prior art designs.
Still another object of the present invention is to provide a pneumatic radial runflat ply tire having good normal inflation handling and ride characteristics as well as good runflat handling properties and runflat durability and operational life.
The present invention relates to a pneumatic radial ply runflat tire having a tread, a carcass comprising a radial ply structure, a belt structure located between the tread and the radial ply structure, an inner liner and two sidewalls each reinforced by a single circumferential wedge insert between the radial ply structure and the inner liner that contains a pair of wedge-insert stiffener layers circumferentially disposed therein. Within each sidewall wedge insert, a first stiffener located closest to the inner liner has a radial width preferably within the range of the range of 30 percent to 50 percent of the total radially oriented span of the respective wedge insert. A second stiffener within each sidewall wedge insert, i.e., the one located most distant from the innerliner, has a radial width that is preferably within the range of between 60 percent to 90 percent and most preferably in the range of 70 percent to 80 percent of the total radially oriented span of the respective wedge insert. The radially innermost and outermost ends of the first wedge-insert stiffener are located radially within the radial span of the second wedge-insert stiffener. Each wedge-insert stiffener comprises parallel-aligned monofilaments, wires or metal cords, which are angularly oriented preferably between 20 degrees and 55 degrees with respect to the radial direction. The parallel-aligned monofilaments, wires or cords of each of the respective two wedge-insert stiffeners within each sidewall wedge are angularly oriented in opposite directions with respect to the radial direction. The wedge-insert stiffeners can be constructed of parallel-aligned nylon or wire monofilaments, or of parallel-aligned metal cords.
Other benefits and advantages of the invention will become apparent to those skilled in the art to which it pertains upon a reading and understanding of the following detailed specification.